An electrophotographic method used for an image forming apparatus, such as a laser printer, and a dry latent electric printer, is typically composed of the following steps (1) to (5).
(1) An image bearing surface, such as a photoconductive layer, is uniformly charged.
(2) The image bearing surface is exposed to light, and the electric charge of the exposed area is dispersed to form an electrically latent image.
(3) A fine powder having electric charge, so-called a toner, is deposited on the latent image to visualize the image.
(4) After transferring the obtained visible image onto a recording medium, such as transfer paper, the image is permanently fixed by heating and pressing.
(5) The fine powder remained on the image bearing surface without being transferred is cleaned.
As for a heating unit or method, a heat roller, an oven, or flash light irradiation is used, and heating temperature is controlled by means of a thermostat or another sensor.
As for a recent image forming apparatus, there is a high demand for energy saving during fixing of a toner, or high-speed processability. Therefore, a toner itself is required to have properties that the toner is melted at low temperature.
In the case where low temperature fixing is achieved by merely reducing a melting point of a toner, however, there is a concern about storage stability of the toner.
Moreover, a demand for high image quality is also high. As for high quality image formation, such as a photographic image, there is a need for providing a vivid high gloss image.
In the aforementioned heat-fixing method, moreover, a surface temperature of a heat roller is controlled depending on the properties of the toner for use, for example when the heat-fixing is performed with the heat roller. In this case, the surface temperature of the heat roller is varied depending on the operation and standing of the heat roller, a feeding state of recording paper, environmental conditions, and overshoot of the heat roller. Therefore, there is a problem that high gloss needs to be achieved regardless of a variation in the fixing temperature.
As for a method for forming a glossy image on an identical recording medium in electrophotography, proposed are a method where gloss is controlled with a number average molecular weight of a resin used in a toner (PTL 1), and a method where releasing properties are enhanced during fixing by encapsulating a release agent in each toner particle (PTL 2).
Moreover, PTL 3 discloses a technology that a gloss control layer is formed on a toner image, where the gloss control layer uses colorless transparent gloss control particles containing a binder resin, and a material that softens the binder resin during heat-fixing.
Furthermore, proposed are methods where gloss is controlled by adjusting viscoelasticity of a toner (PTL 4 to PTL 10). Among these methods, PTL 5 teaches that gloss is imparted by softening the colorless transparent gloss control particles during fixing to level an image surface.
As described above, there are various methods for controlling gloss on the recording medium. For example, PTL 1 teaches that smoothness is increased, and gloss of a clear toner part is partially increased by using a polyester resin having a number average molecular weight of about 3,500 in a clear toner and a polyester resin having a number average molecular weight of about 10,000 in a color toner, and adjusting the melting point of the clear toner lower than the melting point of the color toner.
As the clear toner is formed at an outermost layer of an image, and is directly in contact with a fixing device, the clear toner is required to have higher hot offset resistance than the color toner. Moreover, the clear toner is formed on an image formed of the color toner, and therefore the toner layer becomes thick. Unless the color toner has high cold offset resistance, stability is insufficient with a combination of the clear toner having a low melting point and the color toner having a high melting point.
In the case where a toner is provided with high hot offset resistance, hot offset is typically prevented by introducing a crosslinking monomer to a resin for use to widen a molecular weight of the resin.
If a crosslinking monomer is introduced, however, flowability is not exhibited due to an influence of an elastic component, a smoothness of a toner surface is impaired, and gloss is low, through the hot offset can be prevented.
Moreover, PTL 2 discloses that a toner uses a styrene-acryl resin in order to disperse a release agent in a polyester resin, and hence provided is a toner containing the release agent having a size that can easily exhibit releasing properties, and having fewer side effects due to the release agent. Moreover, it is disclosed that reduction in gloss is prevented by using a certain type of acryl in the resin.
However, spot high glossiness close to photographic gloss, which has been realized by spot varnish, has not yet been realized.
In the method disclosed in PTL 3, a material, which softens the binder resin of the gloss control particles, has a low melting point. Therefore, there is a case where a resulting toner has insufficient storage stability.
Moreover, PTL 4 to PTL 10 disclose that the high gloss is realized when a loss tangent, represented by loss elastic modulus (G″)/storage elastic modulus (G′)=loss tangent (tan δ), has the maximum peak at 80° C. to 160° C., as the viscoelasticity of the toner is measured, and the maximum peak value of the loss tangent is 3 or greater. However, PTL 4 to PTL 10 do not disclose whether or not there is a width in the fixing temperature exhibiting high gloss.